Currently, most high selectivity gas analyzers are based on correlational spectroscopy, wherein gas concentration is determined from characteristic features of the spectrum of the gas under study, such as quasiperiodic absorption or transmission spectra. These gas analyzers should be able to perform multicomponent analysis of the gas under study without unduly complicating their design and should also feature simplicity of conversion to measurement of the content of another gas component in the mixture.
Known in the art is a correlational gas analyzer ("Prikladnaya infrakrasnaya spektroskopiya", Ed. D. Kendall, Moscow, MIR Publishers, 1970), comprising a light source and sequentially positioned along the beam path interference filter to select the specified spectral band of the gas under study, a modulator, two cells with one cell filled with the gas under study and the second cell filled with a gas that does not absorb radiation in the specified spectral band, a photoreceiver, and a recorder.
After passing through the cell filled with the gas under study the light beam does not contain spectral components corresponding to absorption lines, while at the output of the other cell it contains all spectral components of the spectral band selected for gas analysis. The photoreceiver generates a signal proportional to light attenuation in the gas under analysis due to absorption and this signal allows detection and assessment of concentration of the gas under study positioned in the beam path between the light source and the photoreceiver.
This known in the art gas analyzer features poor accuracy and low reproducibility of measurement results due to absorption of the gas under study and to its leakage out of the cell. In case of corrosive gases, such as H.sub.2 S or SO.sub.2, and unstable gases, such as NO.sub.2, the use of this gas analyzer is hampered by the necessity to maintain constant temperature and humidity.
Also known in the art is a correlational gas analyzer (R. Haulet, C. Vavasseur "Teledetection des pollutants gaseoux de l'atmosphere" Bull. inform. sci. ez techn., 1978, 230/231, p.59) comprising a light source with the beam thereof passing through the gas under study which features a quasiperiodic spectrum pattern in the specified spectral band, and through an optical system comprising sequentially positioned along the optical path condensor, input slit iris, specified spectral band dispersing element, and rotatably mounted output slit iris configured as a disc with a group of slits for scanning the specified spectral band of the gas under study across the photoreceiver, with the output thereof connected to a recorder via an electric signal amplifier.
In this known in the art correlational gas analyzer a concave grating is used as the specified spectral band dispersing element, while the slits in the disc are shaped as arcs and positioned to coincide with the maxima and minima in the absorption of the gas under study in the focal plane of the concave grating.
The light passed through the gas under analysis is decomposed into a spectrum by the concave grating and then passed via the slits of the rotating disc. Thus the spectral band of the gas under study is discretely scanned across the photoreceiver, the modulation depth of the light beam being proportional to the difference in intensity of corresponding light transmission and absorption bands in the spectral band of the gas under study and dependent therefore on the content of this gas in the volume.
The optical system of this known in the art gas analyzer is complicated and therefore hard to manufacture, as is disc alignment, because its slits have to be precisely aligned to the maxima and minima of absorption in the gas under study spectrum, this critically affecting the measurement accuracy and leads to low measurement result reproducibility. Conversion to measurements of another gas component of the multicomponent gas mixture requires replacement of the disc with another, with appropriate slits, and its realignment in the optical system.